This invention relates to a method for manufacturing leaf such as laminated springs for automobiles.
At present, leaf springs for automobiles are required to be lighter along with the requirement for lighter automobiles. In the laminated spring, if the achievement of this object is intended only by decreasing the weight without considering the material quality, the durability and fatigue resistance of the leaf spring decreases in proportion to the weight reduction. In order to carry out the reduction of weight it has been attempted to improve hardness of the laminated leaf springs.
However, simply improving hardness of a leaf spring has decreased its tear and impact resistances sometimes shortening the useful life of the spring because of unexpected tear due to impact caused by hard objects such as stones. Accordingly, the improvement of hardness of leaf springs does not offer any solution for reduction of weight.
Then a heat treatment called "ausforming process" or "modified ausforming process" has been applied. In this case, the ausforming process means a heat treatment wherein a raw material of leaf springs is hardened in a hot bath at 300.degree. C. to 600.degree. C. after being heated up to a temperature in the austenite range, then rolled after the temperature of the material has become stable and then quenched so as to obtain fine martensitic microstructure. The modified ausforming process is a heat treatment wherein a raw material is rolled after being heated up to the austenite range temperature and suddenly quenched in an oil bath so as to obtain a fine martensitic microstructure.
In these heat treatment processes, leaf springs resisting against tear and impact have been obtained, but these quenching processes have to be applied soon after being rolled. However, the raw material is hardened so much that an eye forming operation and clip hole and center hole drilling operation become inapplicable or extremely difficult after being quenched, thereby sacrificing productivity.
In this case, only a drilling operation with a bit is applicable for drilling through holes for fixing a center bolt, a plurality of leaf spring and clips, but the drilling operation is impossible because of the hardness. Accordingly, the ausforming process and the modified ausforming process have not been sufficient as a method for manufacturing leaf springs.